Geminate radical pair primary

Primary geminate Secondary geminate radical pair radical pair... 

Photochemical excitation results in a-cleavage to produce a primary, geminate radical pair, which may undergo radical combination reactions (1) in competition with decarbonylation or (2) to produce a secondary geminate radical pair. The latter may undergo radical combination (3) or produce a free-radical pair (4). The free radicals undergo radical combination reactions (5). 

Consider the primary geminate pair (ACO/B) for the specific case of dibenzyl ketone (Figure 9). If the primary geminate radical pair can rotate within the 100 ns time window allowed by the rate of decarbonylation, the carbonyl fragment can attach itself to the ortho or the para position of the benzyl radical, to yield oDBK and pDBK, respectively. If the primary radical pair can diffuse apart and remain apart for the 100 ns, decarbonylation occurs and a secondary geminate radical pair (A/B) is produced. If the mechanistic ideas are correct, the ratio of oDBK to pDBK provides a simple probe of the rotation degrees of freedom available to the primary radical pair. 

The rate of reaction of methyl radicals is in excellent agreement with the predictions of the Smoluchowski theory (see Chap. 2, Sect. 2.6). Consequently, it appears that geminate radicals move towards and away from each other at a diffusion-limited rate. Once an encounter pair is formed, reaction is very rapid (primary recombination). Furthermore, the encounter pair is held together for a considerable time (< 0.1ns in mobile solvents) because the surrounding solvent molecules hinder their separation (solvent caging). There is much evidence which lends some support for this view the most important influences on the recombination probability are listed below. 

The identity and reactivity of this primary product have been interpreted by various models. Only a few important features will be outlined at this point. The radical pair Co L / X- is assumed to be formed in a cage of solvent molecules. Geminate recombination leads back to the starting complex. 

Here quantum yield of 3Chl and

radical products from a 3Cbl... A geminate pair in the primary photochemical act. Particular stages of reaction scheme (5)- (9) will be discussed in more detail in Sects. 3 and 4. 

Within this scheme the quantum yield (O) of PET across the membrane is determined by the product of the yield of the triplet state ( T), fraction of 3Ru(bpy)3+ molecules quenched by C16V2+(cpq), yield of radical-ion products from the geminate pair in the primary photochemical act ([Pg.22]

The secondary electron-transfer processes, often used in mechanistic in estiga-tions and in preparative applications of electron-transfer photochemistry, enhance the quantum yields of product formation [167], In fact, as we have already pointed out in a previous section, the efficiency of separation of the geminate pair is strictly dependent on the redox potentials (see also indirect photooxygenation processes) [43, 50, 80-83, 135], Anyway, although in the present case the subsequent electron-transfer from epoxide to biphenyl radical cation BP is endothermic enough, in the absence of very fast competing reactions this primary radical cation may still undergo an endothermic electron-transfer process with epoxides. 

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